In an exhaust passage of this type of internal combustion engine, there is placed a three-way catalyst which can simultaneously purify three components of HC, CO and NO in an exhaust gas. It is generally considered that the three-way catalyst cannot demonstrate an appropriate purification ability, below activation temperature. Thus, at the start of cooling the engine or the like, a so-called catalyst warm-up operation is performed in which the temperature of the exhaust gas passing through the catalyst is increased so that catalyst temperature reaches the activation temperature at an early stage. During the catalyst warm-up operation, the temperature of the catalyst is monitored, and the warm-up operation is stopped at a time point at which the temperature of the catalyst reaches the activation temperature (e.g. refer to Patent document 1).
It is also known that the three-way catalyst deteriorates due to long hours of purification. Thus, in a hybrid vehicle, there is proposed a technology in which a reducing agent is supplied to the three-way catalyst by forcibly rotating the internal combustion engine with a motor (e.g. refer to Patent document 2).
On the other hand, for example, in order to improve fuel efficiency, a fuel cut technology is used in which the supply of fuel to the internal combustion engine is temporarily stopped (e.g. refer to Patent document 3). If the fuel cut is performed, a reduction in purification ability can be supposedly suppressed by making an air-fuel ratio rich when a fuel stop period exceeds a predetermined period (e.g. refer to Patent document 4). Moreover, if the temperature of the catalyst is high, the deterioration of the catalyst can be supposedly suppressed by temporarily not performing the fuel cut (e.g. refer to Patent document 5).